We have developed a simple in-vitro procedure in which epithelial cells alone or in combination with stroma proliferate as a tissue of many cell layers on a diaphragm of collagen, where both attachment and complete nutrient exchange including oxygen take place at the same surface. Our term "histophysiologic metabolic gradient" refers to those organized arrangements of stratified epithelial cells that have their proliferative function mainly adjacent to the stromal-epithelial interface and their function of maturation with attendant synthesis of specialized products of maturation adjacent to the opposite surface. The extent to which oderly orientation and polarization are in disarray serves as an essential criterion for the surgical pathologist in the evaluation of lesions that may be preinvasive cancer. To the best of our knowledge we are the first to be concerned with the formation in vitro of multilayered sheets of epithelium under histophysiologic conditions, where attachment and complete nutrient exchange take place on the same surface of the epithelium. We are using our procedure, histophysiologic gradient culture, to examine several types of abnormal urothelium, particularly low grade clinical bladder cancer obtained on transurethral resection. We hope to identify, among tumors of similar stage and histopathology, differences in patterns of growth in culture that will disclose predictive differences for individual patients in the neoplastic potential of residual abnormal urothelium.